Emerging clean fuels with high octane rating make spark ignition (SI) technology a promising candidate for heavy-duty applications. The conversion of existing diesel engines to SI operation can accelerate the adoption of these fuels. This study investigates the combustion characteristics of a 500 kWe marine lean-burn (LB) homogeneous charge SI engine with a flat cylinder head and a hemispherical bowl-in piston. It focuses on the relationship between fuel distribution and phasing across the distinct bowl-in and squish combustion phases and their impact on efficiency and emissions in multicylinder engines. The effects of air excess ratio, spark timing, and intake air temperature are systematically assessed. Dedicated measurements of methane and total unburned hydrocarbon emissions enable a comprehensive evaluation of combustion performance and emissions. Results confirm the presence of a slower squish phase, differing from conventional SI engines, and highlight the influence of the squish region's surface-to-volume ratio on flame propagation. The sensitivity of combustion behavior to control parameters such as air excess ratio and ignition timing is demonstrated, with notable differences: while richer mixtures advance bowl-in and squish phases, earlier ignition timing delays the squish phase. Despite this, both mixture enrichment and ignition timing advancement improved performance, increasing brake thermal efficiency by 25% and 10%, respectively. Methane emissions remained within typical ranges for marine SI engines and NOx emissions met Tier III limits at nominal conditions; yet the persistent challenge of methane slip underscores the need for more comprehensive regulatory standards addressing both CH4 and NOx emissions.
